Flecking of fat-filled milk powders

Flecking is an insolubility issue in fat-containing milk powders. The undissolved particles (flecks) are of different shapes and structures, primarily composed of fat and/or protein. The occurrence of flecking in reconstituted milk powders negatively impacts the visual appearance and overall quality of the final product, thereby influencing consumer acceptance and brand trust. Standard quality control measures, like wettability or insolubility, and analysis including rehydration testing are important but not sufficient in predicting, identifying and/or quantifying flecking, often necessitating additional measures to be implemented. Suitable additional analyses for flecking include confocal laser scanning microscopy, electron microscopy, particle size, and density analysis. However, it is crucial to highlight that merely tightening quality control parameters is insufficient to combat flecking. This approach does not allow for the implementation of rapid solutions when the issue is detected at the final stages of quality assessment. To effectively address fleck formation, it is necessary to scrutinize unit operations and identify precisely where, and how, in the process flecks are formed. The issue often requires reformulation and/or engineering interventions, making the final product more robust and resilient to fleck formation. To date, protein denaturation/aggregation and emulsion instability are proposed as major mechanisms governing fleck formation. Additionally, the effect of seasonality of milk chemical composition and reconstitution medium (water/coffee/tea) are other important factors. This work aims to review flecking in reconstituted fat-filled milk powder solutions by interrogating the production process, including the skim milk base wet and dry processing, alongside the powder storage conditions and reconstitution methods, and thereby identify strategies for the control of flecking.